The present invention relates to improvements in a coil spring and particularly to improvements in the construction of the cross-sectional shape of a strand which forms such spring.
A coil spring is usually formed of a strand of circular cross-section, presenting a problem that when it is subjected to axial loads, a maximum stress is produced at a point on the inner peripheral side of the coil, resulting in cracks being formed thereat which lead to breakage.
To solve said problem, there has been proposed a system (hereinafter referred to as a multi-arc system) in which the strand is formed to have a substantially oval cross-sectional shape defined by putting together a plurality of arcs of different radii of curvature.
In said multi-arc system, the radii of curvature of arcs on the various portions of the cross-sectional circumference are changed in such a manner as to lower the maximum stress in consideration of the distribution of stresses in the circular cross section; thus, there are observed improvements over products which use strands of circular cross section.
However, in said multi-arc system, since the radii of curvature of arcs which define the various portions of the cross-sectional circumference of the strand differ stepwise from each other, the distribution of surface stresses is discontinuous at the boundaries of adjacent arcs, tending to produce cracks due to stress concentration.
Generally, the service life of springs is determined by the maximum stress on the cross-sectional circumference of the strand, and in this connection said multi-arc system is still insufficient in lowering the maximum stress on the cross-sectional circumference of the strand.